A considerable variety of techniques and apparatus have been developed over the years for holding films flat for the purpose of enlarging and scanning images.
For example, U.S. Pat. No. 2,248,646 discloses such an apparatus for photographic film tensioning. This patent illustrates in its FIGS. V, VI and VII how the clamp is engaged and how the arched bow springs provide tension to the film strip in one direction.
U.S. Pat. No. 4,965,632 also illustrates a film clamp which tensions film only in one direction using arms which move during clamping. The uniformity of the clamping and tensioning force along the clamped surface is dependent on the spring stiffness of each arm to be very close, and the height of the clamping cylinders to be set up accurately.
FIGS. 1 through 4 illustrate in free body diagram form how a film clamp according to U.S. Pat. No. 4,965,632 functions. The film 1 is positioned on a platen 3 over a rectangular aperture 5. In each of the figures force vectors are indicated by arrowed lines.
In the clamp of this patent, (a) the gripping member that makes contact with the proximal side of the film 1 presses at least essentially vertically against the film 1, and (b) the gripping member that makes contact with the distal side of the film 1 presses both toward the platen 3 and away form the proximal gripping member as illustrated by an arrow 9 of force F.sub.2 in FIG. 4.
Note that, if .mu. is the coefficient of static friction between the film 1 and the platen surface, the following relationship must be maintained: EQU F.sub.2 cos .theta.&lt;.mu.F.sub.1 +.mu.F.sub.2 sin .theta.
Failure to meet this condition will result in slippage of the entire film 1 in the y direction when the force F.sub.2 is applied.
In another patent, U.S. No. 4,161,365 a neghold assembly clamps a film strip between two insert plates. FIGS. 3, 4, and 7 of that patent illustrate the concept. The top insert plate 28 is allowed to pivot in one direction by guides 68 and 70. This provides some ability for the top plate 28 to be clamped parallel to a bottom plate 30. No method for tensioning the film is provided in the patent.
In the prior art film clamp, illustrated in FIGS. 1 and 2, the film 1 is clamped against the platen 3 on all four edges of the rectangular aperture 5 by a vertical force (represented by a plurality of arrows 7). The natural curl in film 1 will not be removed during clamping, and film will not be held flat.
U.S. Pat. No. 4,736,228 also provides a means to clamp film and to maintain parallel clamping plates. However, the design depends on the two springs 12, as shown in FIG. 3 to have very similar stiffness and preload. This patent also provides no tensioning of the film.
In U.S. Pat. No. 5,073,022 tension is applied to a film around the circumference of the aperture opening when the film is clamped between two o-rings that are biased to tension outward from the center of the film by the forming of an incline in the plates that receive the o-rings for clamping. To function properly the rubber o-ring needs to be formed with a uniform thickness, tension and hardness for this technique to provide a flat datum plain for the film. Also the tensioning force will vary based on the above variations. No technique is shown to provide an even clamping force.
Finally, U.S. Pat. No. 2,824,491 illustrates a film holder which is designed to tension the film flat. It also tensions the film in only one direction and its clamping and tensioning force on each edge is dependent on similar spring manufacturing capabilities.
While film holders and clamps of the types just discussed have achieved some acceptance and success, a need for a simple clamping mechanism which provides tension to the film in two directions is needed to achieve uniform flatness.
Also, there is a need for a simple mechanism which applies a uniform clamping and tensioning force around the perimeter of the film. Finally, a mechanism which can be easily converted to different format sizes is desirable.